128 CRUISE OF THE BARRERA 



Between the outer ends of the islands and the 

 coral reefs, a distance varying from one to five 

 miles, is a comparatively open water-way — the 

 "fairway." Through the fairway the alternating 

 tides sweep east and west, keeping the channels 

 scoured out, save for occasional accumulations of 

 coral detritus and sand in eddy spots where sand- 

 bars, partially exposed, have formed by process 

 of tidal action. These little outer islands, sur- 

 rounded by elongated east and west bars, are 

 composed of coarse hard-packed sand, broken 

 fragments of coral, and vast quantities of broken 

 shells, and are generally devoid of vegetation. 

 Within and through the surrounding sand fiats are 

 numerous deep channels with steep banks cut by 

 supplemental tidal action. The depth of these 

 passages is about the same as the floor of the fair- 

 way. It is, of course, obvious that these sand 

 islands, which usually lie just within the reef, 

 have nothing whatever in common with the man- 

 grove islands already referred to. Their formation 

 in all cases is likely due to some original coral 

 obstructions on the bottom which offered a trap 

 for the tide-swept detritus of the reef and which 

 were further aided by the fact that they happened 



